The basal lamina has been ultrastructurally characterized as a continuous electron lucent layer (the lamina lucida) adjacent to the cell surface with an overlying electron dense layer (lamina densa), which interfaces with the mesenchymal stroma (collagens and other matrix proteins). Biochemically, the basal lamina is known to contain type IV collagen, laminin, and basement membrane proteoglycan. The actual disposition of these constituents in the l. lucida l. densa, cell surface, and matrix is uncertain, various conflicting ultrastructural studies notwithstanding. Also, the relationship of type V collagen, a so-called cell surface collagen, to the basal lamina, is unknown. We are employing high resolution (ca. 5 nm) immunoelectron microscopy on tissue sections with purified antisera to laminin, type IV collagen, and type V collagen, using appropriate controls, to precisely localize these constituents of the basal lamina and neighboring extracellular matrix.